Television broadcasters use various time compression techniques to maximize the amount of available advertising time within a particular time period. One such technique is speeding up the playback of a taped production by playing the tape faster than normal. For example, a 3% increase in playback speed of a 30 minute syndicated television program provides a 60 second increase in the amount of time available for advertising. The visual impact of the tape speed increase is not readily discernible to the average viewer.
The above technique may be implemented by coupling the output signal from an accelerated tape player to a frame synchronizer, illustratively a first-in, first-out (FIFO) memory which is read at a normal rate. Eventually the FIFO fills up and a field or frame of video is discarded (i.e., "dropped").
The Moving Pictures Experts Group (MPEG) has promulgated several standards relating to digital data delivery systems. The first, known as MPEG-1 refers to ISO/IEC standards 11172, incorporated herein by reference. The second, known as MPEG-2, refers to ISO/IEC standards 13818, incorporated herein by reference. A compressed digital video system is described in the Advanced Television Systems Committee (ATSC) digital television standard document A/53, incorporated herein by reference.
It is seen to be desirable to perform a speedup or similar function in an MPEG or ATSC environment such that additional advertising time may be utilized by, e.g., television broadcast and cable companies. Current practice for performing a speedup operation on bitstreams simply involves decoding the bitstream, running the decoded bitstream through a Frame Sync (which drops frames as appropriate), and then re-encoding the stream into MPEG form. Unfortunately, removing bits from the bitstream changes a number of bitstream parameters, including the number of bits in the bitstream. These changes may produce undesirable MPEG or ATSC non-conformities, such as buffer synchronization disruption, video buffering verifier (VBV) overflow and VBV underflow.
Therefore, a need exists in the art for a method and apparatus for performing speedup or similar function in an MPEG or ATSC environment without causing undesirable MPEG or ATSC non-conformities.